Acoustics of HVAC Equipment and Environmental Impact on NC Ratings

Acoustics of HVAC Equipment and Environmental Impact on NC Ratings

By Aaron Sorenson – Bosch Applications Engineering

Unwanted noise makes a space uncomfortable and hinders concentration. Excessive noise and vibration are two of the most prevalent complaints from occupants, besides temperature control, and is best dealt with while designing the space. Our objective with this paper is to help the design engineer better understand the basics of acoustics and become familiar with different treatments, methods, or equipment changes that can be applied to the HVAC system to reduce unwanted noise and vibration.

Fundamentals of Acoustics

Noise can be defined as “unwanted or undesired sound, which interferes with speech, concentration, or sleep.” To control the noise, we need to know the frequency (the speed of the vibrations) and amplitude (loudness) of the sound wave(s).

Sound Power and Sound Pressure

Sound power (Lw) is the acoustical energy that a source radiates and is measured in watts. It is a fixed property of the HVAC equipment and not affected by the environment. It cannot be measured directly, except by using special Acoustic Intensity techniques (as determined in an acoustics laboratory and using specific standards to ensure data uniformity). Sound pressure (Lp) however, is affected by the distance to the source as well as the room size, construction materials, absorption, and reflection off surfaces. Sound pressure is what the ear hears.

The brightness of a room could be related to sound pressure, as it’s dependent on the distance to the observer, color of the wall, or shade on the lamp. Whereas the wattage rating would relate to the sound power.

Sound Measurement

Our ears can hear sounds from 20 to 16,000 Hertz (Hz), but to keep the amount of data generated more reasonable, the sound spectrum is divided and captured in 8 octave bands with center frequencies of 63, 125, 250, 500, 1000, 2000, 4000, and 8000 Hz. While it’s practical to compress the range of frequencies together into octave bands, it does mean that some of the tonal “sound quality” characteristics are sacrificed - usually though, the use of octave bands is sufficient for rating the acoustical environment.

The ear can also hear an extensive range of volumes as well; because of this, we use the logarithmic (decibel - dB) scale. The lowest sound pressure that is possible to hear is 2 x 10-5 Pa (20 micro Pascal) and is given the value of 0 dB. The loudest sound (at the threshold of pain, 140 dB) corresponds to a pressure of 200 Pa) which is 10 million times greater than the quietest perceptible sound.

To simplify things, single-number descriptors are typically used for the sound targets for different building environments. To get to single-number descriptors, one must use logarithmic addition. While converting the octave band sound data into single number loses additional frequency information, it is widely used to specify acoustical requirements.

Acoustical Design

To calculate sound pressure, the sound power of the equipment is used along with specific transfer formulas that define how the sound pressure is absorbed or reflected off the surfaces in the space. The conversion details (using AHRI 260 & 350) are unique to the materials used in the space and when applied, result in an application specific sound pressure level.

Sound Absorption

 Influenced by material density, thickness, fiber size, & diameter  Porous materials are better for high frequencies  Denser and thicker materials are better for lower frequencies  Tip: Use a porous absorbing material with a solid backing material to absorb and block sound from transmission Courtesy of http://www.technature.ca/acoustics-101/sound-absorption/

Noise Rating Methods

There are several rating methods used to describe the level of sound or noise within a space. The more common of the methods are A-weighted, C-weighted, NC, and RC. Each of these methods combine the octave-band sound data into a single-number descriptor.

NC Guidelines

The A-weighted rating is corrected for the sensitivity of the human ear with the most emphasis on 1 kHz to 4 kHz while at lower frequencies, the human ear is not as sensitive. It is commonly used for outdoor sound sources, such as highway traffic or outdoor equipment.

The C-weighted scale has less filtering and is commonly used to measure loud sound levels.

The NC “Noise Criteria” curves represent approximately equal loudness levels to the human ear and define the limits that each octave band spectrum must not exceed. NC-35 is the most common noise level specified in HVAC. An NC between 35 and 45 is best for speech privacy in offices.

Case Study Construction Details

In this case study, we will use a school classroom with a 2.5-ton unit and see how the environment affects the sound pressure in the space. The school classroom in this example is 25 feet wide by 25 feet deep with 12-foot-high ceilings. The distance of the ceiling from the occupant is 5 feet.

The classroom is constructed of a mineral fiber ceiling (1lb/ft2 density 5/8-inch-thick), Terrazzo floor, and has average construction (few acoustic leaks / visible holes). The unit is a 2.5 ton horizontal, running in full load cooling, and is connected to 25 feet of 10”x10” duct work.

Standard Unit

To the right, are the remaining details of the space and unit – we will change these parameters later.

We use our handy sound calculator to take the sound power levels of the equipment and subtract or add the attenuation or propagation effects of the construction materials to calculate the sound pressure levels the student will hear. We then apply the NC decibel filter to arrive at a final NC level.

In this case, our sound calculator gives us an NC of 45 with a dBA of 44. This is with duct losses. NC 45 is high for a classroom.

Typically, we would expect a specification of NC 25-30. And maximum of 35.

Extra Quiet

Now we look at what happens when we add the Extra Quiet option.

The extra quiet option adds a compressor blanket and changes the fiber glass insulation on the unit to closed cell foam to give us a dBA of 43 (with duct losses).

We see minimal difference with this option – only 1dBA less (which is not perceptible). The NC remains at 45.

Whisper Quiet

This time we change to a Whisper Quiet QV Unit – everything else stays the same.

Changing to Whisper Quiet made a big difference. The NC Rating dropped from NC45 to NC35 and the dBA went from 43 to 39.

While this is good, it isn’t quiet enough for the most stringent requirements. For that, we need to get down to an NC of 25-30.

Whisper Quiet With Fiberglass Walls

To accomplish this, we look at what would happen if the school classroom walls were constructed with 2x4 framing and 2” 3lb/ cu ft fiberglass insulation instead of painted concrete block.

We find the fiberglass walls make a significant reduction as we go from NC35 to an NC25. The dBA of 39 dropped down to dBA 29. And now we meet some of the most stringent NC requirements for classrooms.

This concludes our case study.

In review, we looked at a few of the more common sound ratings and discussed how some materials can act to absorb or reflect sound more effectively than others. We explored in our case study how a design engineer could do a rough estimation of the resulting sound pressure by having the unit’s sound data and knowing a few things about the construction of the space.